package Stack;

public class MyQueue {
    static class ListNode{
        public int val;
        public ListNode prev;
        public ListNode next;

        public ListNode(int val){
            this.val=val;
        }
    }

    public ListNode first=null;
    public ListNode last=null;
    public int usedSize=0;

    public void offer(int val){
        ListNode cur=new ListNode(val);
        if(isEmpty()){
            first=last=cur;
        }
        last.next=cur;
        cur.prev=last;
        last=last.next;

        usedSize++;
    }
    public int poll(){
        if(isEmpty()){
            return -1;
        }
        int val=first.val;
        first=first.next;
        if(first!=null) {
            first.prev = null;
        }
        usedSize--;
        return val;
    }
    public int peek(){
        if(isEmpty()){
            return -1;
        }
        return first.val;
    }

    public boolean isEmpty(){
        return usedSize==0;
    }
}
